When using a cipher with RSA authentication, an ephemeral DH key exchange
can take place. Ciphers with DSA keys always use ephemeral DH keys as well.
In these cases, the session data are negotiated using the
ephemeral/temporary DH key and the key supplied and certified
by the certificate chain is only used for signing.
Anonymous ciphers (without a permanent server key) also use ephemeral DH keys.

Using ephemeral DH key exchange yields forward secrecy, as the connection
can only be decrypted, when the DH key is known. By generating a temporary
DH key inside the server application that is lost when the application
is left, it becomes impossible for an attacker to decrypt past sessions,
even if he gets hold of the normal (certified) key, as this key was
only used for signing.

In order to perform a DH key exchange the server must use a DH group
(DH parameters) and generate a DH key. The server will always generate
a new DH key during the negotiation.

As generating DH parameters is extremely time consuming, an application
should not generate the parameters on the fly but supply the parameters.
DH parameters can be reused, as the actual key is newly generated during
the negotiation. The risk in reusing DH parameters is that an attacker
may specialize on a very often used DH group. Applications should therefore
generate their own DH parameters during the installation process using the
openssl dhparam(1) application. This application
guarantees that strong primes are used.

Files dh2048.pem, and dh4096.pem in the apps directory of the current
version of the OpenSSL distribution contain the SKIP DH parameters,
which use safe primes and were generated verifiably pseudo-randomly.
These files can be converted into C code using the -C option of the
dhparam(1) application. Generation of custom DH
parameters during installation should still be preferred to stop an
attacker from specializing on a commonly used group. Files dh1024.pem
and dh512.pem contain old parameters that must not be used by
applications.

An application may either directly specify the DH parameters or
can supply the DH parameters via a callback function.

Previous versions of the callback used is_export and keylength
parameters to control parameter generation for export and non-export
cipher suites. Modern servers that do not support export ciphersuites
are advised to either use SSL_CTX_set_tmp_dh() or alternatively, use
the callback but ignore keylength and is_export and simply
supply at least 2048-bit parameters in the callback.